1. Field of the Invention
The invention relates to the technical field of I/Q imbalance correction and, more particularly, to an apparatus and method for adaptively correcting I/Q imbalance.
2. Description of Related Art
Currently, direct conversion receivers are commonly used in portable wireless communication systems due to the features of lower power consumption and better integration, thereby saving the power and reducing the required size. However, such a receiver structure requires overcoming the influence of I/Q imbalance. As shown in FIG. 1, for in-phase signal (I) and quadrature-phase signal (Q) of a received signal Rx, the I/Q imbalance includes a phase imbalance θ and an amplitude imbalance ε. Accordingly, due to the I/Q imbalance, a phase difference between an imbalanced in-phase signal I′ and an imbalanced quadrature-phase signal Q′ cannot remain in orthogonal, i.e., at 90-degree phase difference. In addition, the amplitude cannot remains in consistence.
The I/Q imbalance may reduce the performance of system transmission, especially to a high-speed transmission that generally uses a modulation of 16QAM or 64QAM and is more sensitive to the I/Q imbalance. FIGS. 2 and 3 show demodulated constellations of an orthogonal frequency division multiplex (OFDM) system with a modulation of 16QAM that is respectively influenced by a phase imbalance and an amplitude imbalance. As shown in FIGS. 2 and 3, in the OFDM system, the I/Q imbalance will cause an inter-carrier interference, in addition to the rotation and deformation of the constellations. For example, in the European digital television (DVB-T/DVB-H) specification, the bit error rate (BER) of entire system exceeds a standard when the phase imbalance is greater than five degrees and the amplitude imbalance is greater than 1 dB. In addition, the I/Q imbalance can change with the factors of temperature, time and selected frequency band. Therefore, a typical signal calibration for correcting the I/Q imbalance may fail due to a change of the I/Q imbalance.
To overcome this, both U.S. Pat. No. 5,321,726 granted to K. Kafada for a “Calibration of vector demodulator using statistical analysis”, and U.S. Pat. No. 5,369,411 granted to T. K. Lisle for an “Imbalance correction of in-phase and quadrature phase return signals” use a set of calibration signals to measure required imbalance values for correction. However, such a calibration cannot provide an in-time correction when the I/Q imbalance changes with temperature, time and selected frequency band. Additionally, U.S. Pat. No. 5,105,195 granted to J. C. Conrad for a “System and method for compensation of in-phase and quadrature phase and gain imbalance” uses a fast Fourier transform (FFT) to convert a signal to frequency domain for a correction of the I/Q imbalance. However, such a method is suitable only for the OFDM system. For a non-OFDM system, an additional FFT circuit is still required. U.S. Pat. No. 6,044,112 granted to J. L. Koslov for a “Method and apparatus for correcting amplitude and phase imbalances in demodulators” can correct the I/Q imbalance in time but is suitable only for a single carrier communication system. Therefore, it is desirable to provide an improved apparatus and method for adaptively correcting I/Q imbalance so as to mitigate and/or obviate the aforementioned problems.